Storage system with consistent termination of data replication across multiple distributed processing modules

ABSTRACT

A first storage system in one illustrative embodiment is configured to participate in a replication process with a second storage system. A first processing module of a distributed storage controller of the first storage system detects a replication failure condition for a given write request received from a host device, and provides a corresponding notification to a second processing module of the distributed storage controller. The second processing module, responsive to receipt of the notification, instructs the first processing module and a plurality of additional processing modules of a same type as the first processing module to suspend generation of replication acknowledgments for write requests received from the host device. Responsive to receipt of confirmation from the first and additional processing modules of their suspended generation of replication acknowledgements, the second processing module instructs the first and additional processing modules to terminate the replication process.

FIELD

The field relates generally to information processing systems, and moreparticularly to storage in information processing systems.

BACKGROUND

Many information processing systems are configured to replicate datafrom a storage system at one site to a storage system at another site.In some cases, such arrangements are utilized to support disasterrecovery functionality within the information processing system. Forexample, an enterprise may replicate data from a production data centerto a disaster recovery data center. In the event of a disaster at theproduction site, applications can be started at the disaster recoverysite using the data that has been replicated to that site so that theenterprise can continue its business.

Data replication in these and other contexts can be implemented usingasynchronous replication at certain times and synchronous replication atother times. For example, asynchronous replication may be configured toperiodically transfer data in multiple cycles from a source site to atarget site, while synchronous replication may be configured to mirrorhost writes from the source site to the target site as the writes aremade at the source site. Source site and target site storage systems cantherefore each be configured to support both asynchronous andsynchronous replication modes.

Conventional approaches to data replication can be problematic undercertain conditions. For example, in performing synchronous replicationin a source site storage system having a distributed storage controller,different data path modules of the distributed storage controller may bemirroring different host writes to the target site storage system inparallel with one another. Such a situation can be problematic in thepresence of a replication failure on at least one of the data paths, inthat it is unduly difficult to coordinate disabling of synchronousreplication functionality across the different data path modules in amanner that preserves target replica consistency without underminingsystem performance.

SUMMARY

Illustrative embodiments provide techniques for consistent terminationof data replication across multiple processing modules of a distributedstorage controller in an information processing system. Such embodimentscan advantageously provide highly efficient termination of a synchronousreplication process in the presence of one or more replication failureconditions in a manner that automatically maintains target replicaconsistency in the presence of potentially dependent mirrored hostwrites. Moreover, such advantages are provided without adverselyimpacting system performance.

These embodiments illustratively include a clustered implementation of acontent addressable storage system having a distributed storagecontroller. Similar advantages can be provided in other types of storagesystems.

In one embodiment, an apparatus comprises a first storage system thatincludes a plurality of storage nodes. The first storage system isconfigured to participate in a replication process with a second storagesystem. Each of the storage nodes of the first storage system comprisesa plurality of storage devices. Each of the storage nodes of the firststorage system further comprises a set of processing modules configuredto communicate over one or more networks with corresponding sets ofprocessing modules on other ones of the storage nodes. The sets ofprocessing modules of the storage nodes collectively comprise at least aportion of a distributed storage controller of the first storage system.

In conjunction with the replication process, a first one of theprocessing modules is configured to detect a replication failurecondition for a given write request received from a host device, and toprovide a notification to a second one of the processing modules of thedetected replication failure condition. The second processing module,responsive to receipt of the notification of the detected replicationfailure condition, instructs the first processing module and a pluralityof additional ones of the processing modules of a same type as the firstprocessing module to suspend generation of replication acknowledgmentsfor write requests received from the host device. Furthermore, thesecond processing module, responsive to receipt of confirmation from thefirst and additional processing modules of their suspended generation ofreplication acknowledgements, instructs the first and additionalprocessing modules to terminate the replication process.

The replication failure condition for the given write request maycomprise a failure to receive in the first processing module a responsefrom the second storage system indicating that the given write requesthas been successfully mirrored to the second storage system. Such areplication failure condition may arise, for example, due to failure ofa communication link between the first and second storage systems.

In some embodiments, each of the sets of processing modules comprisesone or more control modules, and at least one of the sets of processingmodules comprises a management module, with the first processing modulecomprising a given one of the control modules, the second processingmodule comprising the management module, and the additional processingmodules of the same type as the first processing module comprisingrespective additional ones of the control modules.

The first and second storage systems illustratively comprise respectivecontent addressable storage systems having respective sets ofnon-volatile memory storage devices. For example, the storage devices ofthe first and second storage systems in such embodiments can beconfigured to collectively provide respective all-flash storage arrays.The first and second storage systems may be associated with respectivesource and target sites of the replication process. For example, thesource site may comprise a production site data center and the targetsite may comprise a disaster recovery site data center. Numerous otherstorage system arrangements are possible in other embodiments.

These and other illustrative embodiments include, without limitation,apparatus, systems, methods and processor-readable storage media.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an information processing system comprisinga content addressable storage system configured with functionality forconsistent termination of data replication across multiple distributedprocessing modules in an illustrative embodiment.

FIG. 2 shows an example of a set of user data pages in an illustrativeembodiment.

FIG. 3 shows an example of a set of metadata pages in an illustrativeembodiment.

FIG. 4 illustrates a portion of a distributed storage controller of acontent addressable storage system showing one possible arrangementsupporting consistent termination of data replication across multipleprocessing modules of the distributed storage controller.

FIGS. 5A and 5B are block diagrams showing different views of aninformation processing system comprising target site and source sitestorage systems configured to participate in a replication process in anillustrative embodiment. These two figures are collectively referred toherein as FIG. 5.

FIG. 6 illustrates interaction between replication engines implementedin respective storage controllers of respective first and second storagesystems as part of a replication process in an illustrative embodiment.

FIG. 7 is a flow diagram of a process for consistent termination of datareplication across multiple distributed processing modules in anillustrative embodiment.

FIGS. 8 and 9 show examples of processing platforms that may be utilizedto implement at least a portion of an information processing system inillustrative embodiments.

DETAILED DESCRIPTION

Illustrative embodiments will be described herein with reference toexemplary information processing systems and associated computers,servers, storage devices and other processing devices. It is to beappreciated, however, that these and other embodiments are notrestricted to the particular illustrative system and deviceconfigurations shown. Accordingly, the term “information processingsystem” as used herein is intended to be broadly construed, so as toencompass, for example, processing systems comprising cloud computingand storage systems, as well as other types of processing systemscomprising various combinations of physical and virtual processingresources. An information processing system may therefore comprise, forexample, at least one data center or other cloud-based system thatincludes one or more clouds hosting multiple tenants that share cloudresources. Numerous other types of enterprise computing and storagesystems are also encompassed by the term “information processing system”as that term is broadly used herein.

FIG. 1 shows an information processing system 100 configured inaccordance with an illustrative embodiment. The information processingsystem 100 comprises a computer system 101 that includes compute nodes102-1, 102-2, . . . 102-N. The compute nodes 102 communicate over anetwork 104 with a content addressable storage system 105. The computersystem 101 is assumed to comprise an enterprise computer system or otherarrangement of multiple compute nodes associated with respective users.

The compute nodes 102 illustratively comprise respective processingdevices of one or more processing platforms. For example, the computenodes 102 can comprise respective virtual machines (VMs) each having aprocessor and a memory, although numerous other configurations arepossible.

The compute nodes 102 can additionally or alternatively be part of cloudinfrastructure such as an Amazon Web Services (AWS) system. Otherexamples of cloud-based systems that can be used to provide computenodes 102 and possibly other portions of system 100 include Google CloudPlatform (GCP) and Microsoft Azure.

The compute nodes 102 may be viewed as examples of what are moregenerally referred to herein as “host devices” or simply “hosts.” Suchhost devices are configured to write data to and read data from thecontent addressable storage system 105. The compute nodes 102 and thecontent addressable storage system 105 may be implemented on a commonprocessing platform, or on separate processing platforms. A wide varietyof other types of host devices can be used in other embodiments.

The compute nodes 102 in some embodiments illustratively provide computeservices such as execution of one or more applications on behalf of eachof one or more users associated with respective ones of the computenodes 102.

The term “user” herein is intended to be broadly construed so as toencompass numerous arrangements of human, hardware, software or firmwareentities, as well as combinations of such entities. Compute and/orstorage services may be provided for users under a platform-as-a-service(PaaS) model, although it is to be appreciated that numerous other cloudinfrastructure arrangements could be used. Also, illustrativeembodiments can be implemented outside of the cloud infrastructurecontext, as in the case of a stand-alone enterprise-based computing andstorage system.

The network 104 is assumed to comprise a portion of a global computernetwork such as the Internet, although other types of networks can bepart of the network 104, including a wide area network (WAN), a localarea network (LAN), a satellite network, a telephone or cable network, acellular network, a wireless network such as a WiFi or WiMAX network, orvarious portions or combinations of these and other types of networks.The network 104 in some embodiments therefore comprises combinations ofmultiple different types of networks each comprising processing devicesconfigured to communicate using Internet Protocol (IP) or othercommunication protocols.

As a more particular example, some embodiments may utilize one or morehigh-speed local networks in which associated processing devicescommunicate with one another utilizing Peripheral Component Interconnectexpress (PCIe) cards of those devices, and networking protocols such asInfiniBand, Gigabit Ethernet or Fibre Channel. Numerous alternativenetworking arrangements are possible in a given embodiment, as will beappreciated by those skilled in the art.

The content addressable storage system 105 is accessible to the computenodes 102 of the computer system 101 over the network 104. The contentaddressable storage system 105 comprises a plurality of storage devices106 and an associated storage controller 108. The storage devices 106are configured to store metadata pages 110 and user data pages 112, andmay also store additional information not explicitly shown such ascheckpoints and write journals. The metadata pages 110 and the user datapages 112 are illustratively stored in respective designated metadataand user data areas of the storage devices 106. Accordingly, metadatapages 110 and user data pages 112 may be viewed as corresponding torespective designated metadata and user data areas of the storagedevices 106.

A given “page” as the term is broadly used herein should not be viewedas being limited to any particular range of fixed sizes. In someembodiments, a page size of 8 kilobytes (KB) is used, but this is by wayof example only and can be varied in other embodiments. For example,page sizes of 4 KB or other values can be used. Accordingly,illustrative embodiments can utilize any of a wide variety ofalternative paging arrangements for organizing the metadata pages 110and the user data pages 112.

The user data pages 112 are part of a plurality of logical units (LUNs)configured to store files, blocks, objects or other arrangements ofdata, each also generally referred to herein as a “data item,” on behalfof users associated with compute nodes 102. Each such LUN may compriseparticular ones of the above-noted pages of the user data area. The userdata stored in the user data pages 112 can include any type of user datathat may be utilized in the system 100. The term “user data” herein istherefore also intended to be broadly construed.

It is assumed in the present embodiment that the storage devices 106comprise solid state drives (SSDs). Such SSDs are implemented usingnon-volatile memory (NVM) devices such as flash memory. Other types ofNVM devices that can be used to implement at least a portion of thestorage devices 106 include non-volatile random access memory (NVRAM),phase-change RAM (PC-RAM) and magnetic RAM (MRAM). Various combinationsof multiple different types of NVM devices may also be used.

However, it is to be appreciated that other types of storage devices canbe used in other embodiments. For example, a given storage system as theterm is broadly used herein can include a combination of different typesof storage devices, as in the case of a multi-tier storage systemcomprising a flash-based fast tier and a disk-based capacity tier. Insuch an embodiment, each of the fast tier and the capacity tier of themulti-tier storage system comprises a plurality of storage devices withdifferent types of storage devices being used in different ones of thestorage tiers. For example, the fast tier may comprise flash driveswhile the capacity tier comprises hard disk drives. The particularstorage devices used in a given storage tier may be varied in otherembodiments, and multiple distinct storage device types may be usedwithin a single storage tier. The term “storage device” as used hereinis intended to be broadly construed, so as to encompass, for example,flash drives, solid state drives, hard disk drives, hybrid drives orother types of storage devices.

In some embodiments, the content addressable storage system 105illustratively comprises a scale-out all-flash storage array such as anXtremIO™ storage array from Dell EMC of Hopkinton, Mass. Other types ofstorage arrays, including by way of example VNX® and Symmetrix VMAX®storage arrays also from Dell EMC, can be used to implement storagesystems in other embodiments.

The term “storage system” as used herein is therefore intended to bebroadly construed, and should not be viewed as being limited to contentaddressable storage systems or flash-based storage systems. A givenstorage system as the term is broadly used herein can comprise, forexample, network-attached storage (NAS), storage area networks (SANs),direct-attached storage (DAS) and distributed DAS, as well ascombinations of these and other storage types, includingsoftware-defined storage.

Other particular types of storage products that can be used inimplementing a given storage system in an illustrative embodimentinclude all-flash and hybrid flash storage arrays such as Unity™,software-defined storage products such as ScaleIO™ and ViPR®, cloudstorage products such as Elastic Cloud Storage (ECS), object-basedstorage products such as Atmos®, and scale-out NAS clusters comprisingIsilon® platform nodes and associated accelerators, all from Dell EMC.Combinations of multiple ones of these and other storage products canalso be used in implementing a given storage system in an illustrativeembodiment.

The content addressable storage system 105 in the embodiment of FIG. 1is configured to generate hash metadata providing a mapping betweencontent-based digests of respective ones of the user data pages 112 andcorresponding physical locations of those pages in the user data area.Such content-based digests are examples of what are more generallyreferred to herein as “content-based signatures” of the respective userdata pages 112. The hash metadata generated by the content addressablestorage system 105 is illustratively stored as metadata pages 110 in themetadata area.

The generation and storage of the hash metadata is assumed to beperformed under the control of the storage controller 108. The hashmetadata may be stored in the metadata area in a plurality of entriescorresponding to respective buckets each comprising multiple cachelines, although other arrangements can be used.

Each of the metadata pages 110 characterizes a plurality of the userdata pages 112. For example, as illustrated in FIG. 2, a given set ofuser data pages 200 representing a portion of the user data pages 112illustratively comprises a plurality of user data pages denoted UserData Page 1, User Data Page 2, . . . User Data Page n. Each of the userdata pages in this example is characterized by a LUN identifier, anoffset and a content-based signature. The content-based signature isgenerated as a hash function of content of the corresponding user datapage. Illustrative hash functions that may be used to generate thecontent-based signature include SHA1, where SHA denotes Secure HashingAlgorithm, or other SHA protocols known to those skilled in the art. Thecontent-based signature is utilized to determine the location of thecorresponding user data page within the user data area of the storagedevices 106 of the content addressable storage system 105.

Each of the metadata pages 110 in the present embodiment is assumed tohave a signature that is not content-based. For example, the metadatapage signatures may be generated using hash functions or other signaturegeneration algorithms that do not utilize content of the metadata pagesas input to the signature generation algorithm. Also, each of themetadata pages is assumed to characterize a different set of the userdata pages.

This is illustrated in FIG. 3, which shows a given set of metadata pages300 representing a portion of the metadata pages 110 in an illustrativeembodiment. The metadata pages in this example include metadata pagesdenoted Metadata Page 1, Metadata Page 2, . . . Metadata Page m, havingrespective signatures denoted Signature 1, Signature 2, . . . Signaturem. Each such metadata page characterizes a different set of n user datapages. For example, the characterizing information in each metadata pagecan include the LUN identifiers, offsets and content-based signaturesfor each of the n user data pages that are characterized by thatmetadata page. It is to be appreciated, however, that the user data andmetadata page configurations shown in FIGS. 2 and 3 are examples only,and numerous alternative user data and metadata page configurations canbe used in other embodiments.

The content addressable storage system 105 in the FIG. 1 embodiment isimplemented as at least a portion of a clustered storage system andincludes a plurality of storage nodes 115 each comprising acorresponding subset of the storage devices 106. Other clustered storagesystem arrangements comprising multiple storage nodes can be used inother embodiments. A given clustered storage system may include not onlystorage nodes 115 but also additional storage nodes 120 coupled tonetwork 104. Alternatively, the additional storage nodes 120 may be partof another clustered storage system of the system 100. Each of thestorage nodes 115 and 120 of the system 100 is assumed to be implementedusing at least one processing device comprising a processor coupled to amemory.

The storage controller 108 of the content addressable storage system 105is implemented in a distributed manner so as to comprise a plurality ofdistributed storage controller components implemented on respective onesof the storage nodes 115 of the content addressable storage system 105.The storage controller 108 is therefore an example of what is moregenerally referred to herein as a “distributed storage controller.” Insubsequent description herein, the storage controller 108 may be moreparticularly referred to as a distributed storage controller.

Each of the storage nodes 115 in this embodiment further comprises a setof processing modules configured to communicate over one or morenetworks with corresponding sets of processing modules on other ones ofthe storage nodes 115. The sets of processing modules of the storagenodes 115 collectively comprise at least a portion of the distributedstorage controller 108 of the content addressable storage system 105.

The distributed storage controller 108 in the present embodiment isconfigured to implement functionality for one or more replicationprocesses carried out between the content addressable storage system 105and another storage system. The term “replication process” as usedherein is intended to be broadly construed, so as to encompass a singlereplication process that includes separate asynchronous and synchronousreplication modes, as well as a replication process that includesmultiple separate asynchronous and synchronous replication processes. Inan arrangement of the latter type, the asynchronous and synchronousreplication processes are also considered examples of what are moregenerally referred to herein as respective asynchronous and synchronous“replication modes.” A given replication process as that term isgenerally used herein can in some cases include either a synchronousreplication mode or an asynchronous replication mode, and no otherreplication modes.

The modules of the distributed storage controller 108 in the presentembodiment more particularly comprise different sets of processingmodules implemented on each of the storage nodes 115. The set ofprocessing modules of each of the storage nodes 115 comprises at least acontrol module 108C, a data module 108D and a routing module 108R. Thedistributed storage controller 108 further comprises one or moremanagement (“MGMT”) modules 108M. For example, only a single one of thestorage nodes 115 may include a management module 108M. It is alsopossible that management modules 108M may be implemented on each of atleast a subset of the storage nodes 115.

Communication links are established between the various processingmodules of the distributed storage controller 108 using well-knowncommunication protocols such as Transmission Control Protocol (TCP) andInternet Protocol (IP). For example, respective sets of IP links used inreplication data transfer could be associated with respective differentones of the routing modules 108R and each such set of IP links couldinclude a different bandwidth configuration.

Ownership of a user data logical address space within the contentaddressable storage system 105 is illustratively distributed among thecontrol modules 108C. The management module 108M may include areplication engine or other arrangement of replication control logicthat engages corresponding replication control logic instances in all ofthe control modules 108C and routing modules 108R in order to implementa data replication process within the system 100, as will be describedin more detail below in conjunction with FIG. 4. The data replicationprocess illustratively involves replicating data from one portion of astorage system to another portion of that system, or from one storagesystem to another storage system. It is desirable in these and otherdata replication contexts to implement consistent termination of areplication process across multiple distributed processing modules, suchas the control modules 108C of the distributed storage controller 108.

In some embodiments, the content addressable storage system 105comprises an XtremIO™ storage array suitably modified to incorporateconsistent termination techniques as disclosed herein. In arrangementsof this type, the control modules 108C, data modules 108D and routingmodules 108R of the distributed storage controller 108 illustrativelycomprise respective C-modules, D-modules and R-modules of the XtremIO™storage array. The one or more management modules 108M of thedistributed storage controller 108 in such arrangements illustrativelycomprise a system-wide management module (“SYM module”) of the XtremIO™storage array, although other types and arrangements of system-widemanagement modules can be used in other embodiments. Accordingly,consistent termination functionality in some embodiments is implementedunder the control of at least one system-wide management module of thedistributed storage controller 108.

In the above-described XtremIO™ storage array example, each user datapage typically has a size of 8 KB and its content-based signature is a20-byte signature generated using an SHA1 hash function. Also, each pagehas a LUN identifier and an offset, and so is characterized by <lun_id,offset, signature>.

As mentioned previously, storage controller components in an XtremIO™storage array illustratively include C-module and D-module components.For example, separate instances of such components can be associatedwith each of a plurality of storage nodes in a clustered storage systemimplementation.

The distributed storage controller in this example is configured togroup consecutive pages into page groups, to arrange the page groupsinto slices, and to assign the slices to different ones of theC-modules.

The D-module allows a user to locate a given user data page based on itssignature. Each metadata page also has a size of 8 KB and includesmultiple instances of the <lun_id, offset, signature> for respectiveones of a plurality of the user data pages. Such metadata pages areillustratively generated by the C-module but are accessed using theD-module based on a metadata page signature.

The metadata page signature in this embodiment is a 20-byte signaturebut is not based on the content of the metadata page. Instead, themetadata page signature is generated based on an 8-byte metadata pageidentifier that is a function of the LUN identifier and offsetinformation of that metadata page.

If a user wants to read a user data page having a particular LUNidentifier and offset, the corresponding metadata page identifier isfirst determined, then the metadata page signature is computed for theidentified metadata page, and then the metadata page is read using thecomputed signature. In this embodiment, the metadata page signature ismore particularly computed using a signature generation algorithm thatgenerates the signature to include a hash of the 8-byte metadata pageidentifier, one or more ASCII codes for particular predeterminedcharacters, as well as possible additional fields. The last bit of themetadata page signature may always be set to a particular logic value soas to distinguish it from the user data page signature in which the lastbit may always be set to the opposite logic value.

The metadata page signature is used to retrieve the metadata page viathe D-module. This metadata page will include the <lun_id, offset,signature> for the user data page if the user page exists. The signatureof the user data page is then used to retrieve that user data page, alsovia the D-module.

Additional examples of content addressable storage functionalityimplemented in some embodiments by control modules 108C, data modules108D, routing modules 108R and management module(s) 108M of distributedstorage controller 108 can be found in U.S. Pat. No. 9,104,326, entitled“Scalable Block Data Storage Using Content Addressing,” which isincorporated by reference herein. Alternative arrangements of these andother storage node processing modules of a distributed storagecontroller in a content addressable storage system can be used in otherembodiments.

The content addressable storage system 105 in the FIG. 1 embodiment isassumed to be configured to participate in a replication process with asecond storage system that is not explicitly shown in the figure. Thecontent addressable storage system 105 is an example of what is referredto herein as a “first storage system” relative to the second storagesystem. In certain description below, the content addressable storagesystem 105 will therefore be referred to as the first storage system.Each of the first and second storage systems comprises a plurality ofstorage devices, such as flash-based storage devices.

The replication process illustratively includes both synchronous andasynchronous replication modes. The synchronous replication modeinvolves mirroring host writes from the first storage system to thesecond storage system, while the asynchronous replication mode utilizescycle-based asynchronous replication. Other types of synchronous andasynchronous replication modes and processes can be used in otherembodiments.

More particularly, in this embodiment, the storage controller of thefirst storage system comprises replication control logic configured tocooperatively interact with corresponding replication control logic in astorage controller of the second storage system in order to execute atleast a synchronous replication process carried out between the firstand second storage systems. The second storage system can be implementedon the same processing platform as the first storage system or on adifferent processing platform. The replication control logic of a givenone of the first and second storage systems may comprise software,hardware or firmware, or combinations thereof, implemented in one ormore storage node processing modules, such as control modules, datamodules, routing modules and management modules of a distributed storagecontroller of the corresponding storage system.

The first and second storage system in this embodiment are furtherassumed to be implemented as respective clustered storage systems eachhaving a plurality of storage nodes implementing a distributed storagecontroller such as distributed storage controller 108 of contentaddressable storage system 105.

Each of the storage nodes of the first storage system comprises a set ofprocessing modules configured to communicate over one or more networkswith corresponding sets of processing modules on other ones of thestorage nodes. A given such set of processing modules implemented on aparticular storage node illustratively includes at least one controlmodule 108C, at least one data module 108D and at least one routingmodule 108R, and possibly a management module 108M. These sets ofprocessing modules of the storage nodes collectively comprise at least aportion of a distributed storage controller of the first storage system,such as the distributed storage controller 108. The storage nodes of thesecond storage system are assumed to be configured in a similar manner.

In conjunction with the replication process, a first processing moduleof a distributed storage controller of the first storage system detectsa replication failure condition for a given write request received froma host device, and provides a corresponding notification to a secondprocessing module of the distributed storage controller. The secondprocessing module, responsive to receipt of the notification, instructsthe first processing module and a plurality of additional processingmodules of a same type as the first processing module to suspendgeneration of replication acknowledgments for write requests receivedfrom the host device. Responsive to receipt of confirmation from thefirst and additional processing modules of their suspended generation ofreplication acknowledgements, the second processing module instructs thefirst and additional processing modules to terminate the replicationprocess.

The term “write request” as used herein is intended to be broadlyconstrued, so as to encompass one or more input-output (TO) operationsdirecting that at least one data item of a storage system be written toin a particular manner. A given write request is illustratively receivedin a storage system from a host device. For example, in someembodiments, a write request is received in a distributed storagecontroller of the storage system, and directed from one processingmodule to another processing module of the distributed storagecontroller. More particularly, in the embodiment to be described belowin conjunction with FIG. 5B, a received write request is directed from arouting module of a source site storage system to a control module ofthe source site storage system. Other arrangements for receiving andprocessing write requests from one or more host devices can be used.

The term “replication acknowledgement” as used herein is also intendedto be broadly construed, so as to encompass any type of update, statusreport or other message that would ordinarily be provided by aprocessing module of a storage system to a host device responsive to awrite request generated by that host device and directed to a data itemthat is subject to replication in the storage system.

In the present embodiment, the first processing module illustrativelycomprises a given one of the control modules 108C of the distributedstorage controller 108, the second processing module comprises themanagement module 108M, and the additional processing modules of thesame type as the first processing module comprise respective additionalones of the control modules 108C. Other arrangements of distributedprocessing modules of a distributed storage controller are possible inother embodiments.

Referring now to FIG. 4, a more detailed view of a portion of thedistributed storage controller 108 in an illustrative embodiment isshown. This embodiment illustrates an example of communications betweencontrol modules 108C and routing modules 108R of the distributed storagecontroller 108.

The management module 108M of the distributed storage controller 108 inthis embodiment more particularly comprises a system-wide managementmodule or SYM module of the type mentioned previously. Although only asingle SYM module is shown in this embodiment, other embodiments caninclude multiple instances of the SYM module possibly implemented ondifferent ones of the storage nodes. It is therefore assumed that thedistributed storage controller 108 comprises one or more managementmodules 108M.

A given instance of management module 108M comprises replication controllogic 400 and associated management program code 402. The managementmodule 108M communicates with control modules 108C-1 through 108C-x,also denoted as C-module 1 through C-module x. The control modules 108Ccommunicate with routing modules 108R-1 through 108R-y, also denoted asR-module 1 through R-module y. The variables x and y are arbitraryintegers greater than one, and may but need not be equal. In someembodiments, each of the storage nodes 115 of the content addressablestorage system 105 comprises one of the control modules 108C and one ofthe routing modules 108R, as well as one or more additional modulesincluding one of the data modules 108D.

The control modules 108C-1 through 108C-x in the FIG. 4 embodimentcomprise respective messaging interfaces 404C-1 through 404C-x. Thesemessaging interfaces 404C are utilized by corresponding instances ofreplication control logic 406C-1 through 406C-x to generate, receive andotherwise process messages in conjunction with a replication process.For example, the messaging interfaces 404C are utilized to generatecontrol-to-routing messages for transmission to the routing modules108R, and to process routing-to-control messages received from therouting modules 108R. The messaging interfaces 404C also generatemessages for transmission to the management module 108M and processinstructions and other messages received from the management module108M.

The replication process is assumed to comprise a synchronous replicationprocess in which write requests directed by one or more host devices tothe first storage system are mirrored to the second storage system. Itis the synchronous replication process that is subject to consistenttermination techniques in illustrative embodiments. When a synchronousreplication process is enabled for a particular data item or set of dataitems, the first storage system mirrors host writes to the data item ordata items to the second storage system as part of handling those hostwrites, and only responds to an initiating host after receivingacknowledgement of successful replication from the second storagesystem.

The replication process can additionally include a cycle-basedasynchronous replication process in which the control modules 108C scandifferences in designated replication data between replication cycles,and send corresponding data transfer requests as needed to the routingmodules 108R. The routing modules 108R in turn replicate the data to aremote storage node cluster of the second storage system, and thenrespond to the control modules 108C regarding the data replicationresults.

The routing modules 108R-1 through 108R-y in the FIG. 4 embodimentcomprise respective messaging interfaces 404R-1 through 404R-y. Thesemessaging interfaces 404R are utilized by corresponding instances ofreplication control logic 406R-1 through 406R-y to generaterouting-to-control messages for transmission to one or more of thecontrol modules 108C and to process control-to-routing messages receivedfrom one or more of the control modules 108C in conjunction with thereplication process.

For example, as indicated above, a given one of the control modules 108Cmay be configured to generate a request message as a control-to-routingmessage for transmission to a given one of the routing modules 108Rrequesting that the given routing module transfer designated replicationdata to the second storage system.

The manner in which consistent termination of a synchronous replicationprocess is provided in the FIG. 4 embodiment will now be described. Itis assumed that a synchronous replication process is currently beingcarried out by the processing modules 108C, 108D, 108R and 108M. Inconjunction with the replication process, a particular one of thecontrol modules 108C detects a replication failure condition for a givenwrite request received from a host device. The host device isillustratively one of the compute nodes 102 of the computer system 101.The particular control module 108C provides a notification of thedetected replication failure to the management module 108M.

The synchronous replication process in this embodiment is assumed to beconfigured such that the second storage system generates for eachsuccessfully mirrored write request a corresponding response back to thefirst storage system. This response generally comes from a routingmodule of the second storage system back to the particular controlmodule that requested the data transfer for mirroring of the writerequest. The requesting control module would then normally provide areplication acknowledgement back to the host device that generated thewrite request, so as to indicate to the host device that the writerequest has been successfully mirrored to the second storage system.

The detected replication failure condition for the given write requesttherefore illustratively comprises a failure to receive in therequesting control module a corresponding response from the secondstorage system indicating that the given write request has beensuccessfully mirrored to the second storage system. For example, thereplication failure condition may be detected upon expiration of aspecified timeout period without the expected successful mirroringresponse being received from the second storage system. The timeoutperiod may be measured from transmission of a data transfer request fromthe requesting control module of the first storage system. Other typesof replication failure conditions and failure detection mechanisms canbe used in other embodiments.

The notification of the detected replication failure condition may beone of a plurality of such notifications received in the managementmodule 108M from respective different ones of the control modules 108C.

Responsive to receipt of the notification of the detected replicationfailure condition, the management module 108M instructs all of thecontrol modules 108C to suspend generation of replicationacknowledgments for write requests received from the host device. Thecontrol modules 108C may each be configured to set a replication barrierresponsive to receipt of the instruction from the management module 108Mto suspend generation of replication acknowledgments for write requestsreceived from the host device. Such a replication barrier illustrativelyprevents further replication of additional write requests by thecorresponding control module.

It should be noted in this regard that different ones of the controlmodules 108C may be receiving write requests from different hostdevices. Multiple host devices may therefore be generating writerequests that are subject to replication to the second storage system ina synchronous replication process. Accordingly, references herein to a“host device” should be broadly construed as potentially encompassingone or more host devices.

Each of the control modules 108C sends a confirmation message back tothe management module 108M indicating that it has suspended generationof replication acknowledgements to the host device.

Responsive to receipt of the confirmation messages from the controlmodules 108C, the management module 108M instructs all of the controlmodules 108C to terminate the replication process. As indicated above,it is assumed for purposes of the present embodiment that thereplication process comprises a synchronous replication process.Accordingly, the first storage system may transition from the terminatedsynchronous replication process back to an asynchronous replicationprocess, or alternatively from a terminated synchronous replication modeto an asynchronous replication mode in a replication process thatincludes both synchronous and asynchronous modes of operation. The firststorage system may subsequently transfer back to the synchronousreplication process or mode of operation from the asynchronousreplication process or mode of operation.

The management module 108M illustratively instructs all of the controlmodules 108C of the distributed storage controller 108 to terminate thesynchronous replication process only after confirmation of suspendedgeneration of replication acknowledgements is received from each ofthose control modules. The management module 108M may instruct all ofthe control modules 108C of the distributed storage controller 108 toterminate the replication process by instructing each of the controlmodules to stop mirroring write requests to the second storage system.It is possible that different ones of the control modules 108C will stopmirroring write requests to the second storage system at differenttimes.

Each of the control modules 108C releases its set replication barrier inconjunction with termination of the replication process. The firstprocessing module may acknowledge the given write request to the hostdevice in conjunction with termination of the replication process.

The above-described operations of the given control module and the givenrouting module are carried out under the control of their respectivecontrol logic instances 406C and 406R in cooperation with thereplication control logic 400 of the management module 108M. The othercontrol logic instances 406C and 406R in the other control and routingmodules 108C and 108R are similarly configured to control messageprocessing in order to implement portions of a replication process asdisclosed herein.

As a more particular example in the XtremIO™ context, a process forconsistent termination of data replication across multiple processingmodules is advantageously configured to automatically maintain targetreplica consistency in the presence of potentially dependent mirroredhost writes. Moreover, such advantages are provided without adverselyimpacting system performance.

The C-modules, D-modules and R-modules of the storage nodes in thiscontext are assumed to be configured to communicate with one anotherover a high-speed internal network such as an InfiniBand (TB) network.The C-modules, D-modules and R-modules coordinate with one another toaccomplish various TO processing tasks.

Communications between the modules may be subject to synchronousmessaging timeout periods configured to ensure that host TO operationswill not time out even if module failure recovery is needed in thedistributed storage controller.

Additionally or alternatively, asynchronous messaging techniques may beused, such as those disclosed in U.S. patent application Ser. No.15/824,536, filed Nov. 28, 2017 and entitled “Storage System withAsynchronous Messaging between Processing Modules for Data Replication,”which is incorporated by reference herein. These asynchronous messagingtechniques can avoid problems that could otherwise result if networkissues cause data transfer between the source and target site storagesystems to take a relatively long time. For example, undesirabletimeouts in the replication data transfer messages exchanged between theC-modules and the R-modules can be more readily avoided.

It is assumed for purposes of the present example that data transferrequest messages can be sent from any C-module to any R-module in thestorage node cluster of the corresponding storage system. A given datatransfer request message sent from a C-module to an R-module willreceive an immediate response within a synchronous messaging timeoutperiod. The response will usually be successful and the received datatransfer request message will be processed by the R-module using abackground thread. An error will be returned if there is something wrongwith the message or if the R-module cannot process the message. Datatransfer result responses will be sent back from the R-module to theC-module as separate messages.

The consistent termination functionality in this particular example ismore specifically implemented as follows.

First, a given C-module 108C-1 detects a replication failure condition.As a result of the detected replication failure condition, the C-module108C-1 gives up on mirroring the corresponding host write to the targetstorage system. The C-module 108C-1 therefore holds the host writewithout acknowledging it to the host. This is to prevent the host fromsending subsequent write requests that may have dependence on the failedwrite.

The given C-module 108C-1 then notifies the SYM module 108M thatsynchronous replication should be disabled. Several different C-modules108C may notify the SYM module 108M independently and substantiallysimultaneously.

Upon receiving the notification from the given C-module 108C-1 thatsynchronous replication should be disabled, the SYM module 108Morchestrates a two-phase algorithm:

Phase I: The SYM module 108M instructs all C-modules 108C to suspendgeneration of write request replication acknowledgements to the host.Upon receiving this instruction message from the SYM module 108M, eachof the C-modules 108C sets a corresponding synchronous replicationbarrier. From this point on, all host writes will be waiting on thesynchronous replication barriers of the respective C-modules 108C andtherefore will not be acknowledged to the host. This guarantees targetconsistency when there is a time lag in switching of replication modebetween the multiple distributed C-modules 108C as part of Phase II ofthe two-phase algorithm. After receiving responses from all of theC-modules 108C indicating that Phase I is complete, the SYM module 108Mmoves to Phase II.

Phase II: The SYM module 108M instructs all C-modules 108C to stopmirroring writes to the target, thereby effectively disabling thesynchronous replication mode in all data path modules. Upon receivingthis instruction message from the SYM module 108M, each of the C-modules108C disables synchronous replication mode. In addition, each of theC-modules 108C releases the synchronous replication barrier it hadpreviously set in Phase I, and so resumes processing of host writes andother IO operations. Also, the original host write that was held in thegiven C-module 108C-1 when the consistent termination procedure wastriggered by detection of a replication failure condition is finallyacknowledged to the host.

The above two-phase algorithm guarantees a write-consistent targetreplica upon synchronous replication mode termination in the distributedC-modules 108C, with each such C-module performing actual mirroringtermination on its own time. Setting of the synchronous replicationbarriers as part of Phase I of the two-phase algorithm allows theconsistent termination procedure to be implemented in a way that avoidsany adverse performance impact on the normal operation of the system.

The example algorithm described above is executed at the source siteutilizing replication control logic instances 400, 406C and 406R of therespective storage node processing modules 108M, 108C and 108R of thefirst storage system.

It is to be appreciated that the particular algorithm steps areexemplary only, and can be varied in other embodiments.

Also, the particular interconnection and signaling arrangementsillustrated for processing modules 108C, 108R and 108M in FIG. 4 arepresented by way of example only, and can be varied in otherembodiments.

In some embodiments, the replication control logic of these processingmodules comprises at least a portion of a replication engine of thestorage controller 108. An example of such a replication engine and itsassociated processing operations will be described in more detail belowin conjunction with the embodiment of FIG. 6.

It should also be understood that the particular arrangement of storagecontroller processing modules 108C, 108D, 108R and 108M as shown in theFIG. 1 embodiment is presented by way of example only. Numerousalternative arrangements of processing modules of a distributed storagecontroller may be used to implement consistent termination functionalityfor data replication in a clustered storage system in other embodiments.

Although illustratively shown as being implemented within the contentaddressable storage system 105, the storage controller 108 in otherembodiments can be implemented at least in part within the computersystem 101, in another system component, or as a stand-alone componentcoupled to the network 104.

The computer system 101 and content addressable storage system 105 inthe FIG. 1 embodiment are assumed to be implemented using at least oneprocessing platform each comprising one or more processing devices eachhaving a processor coupled to a memory. Such processing devices canillustratively include particular arrangements of compute, storage andnetwork resources. For example, processing devices in some embodimentsare implemented at least in part utilizing virtual resources such as VMsor Linux containers (LXCs), or combinations of both as in an arrangementin which Docker containers or other types of LXCs are configured to runon VMs.

As a more particular example, the storage controller 108 can beimplemented in the form of one or more LXCs running on one or more VMs.Other arrangements of one or more processing devices of a processingplatform can be used to implement the storage controller 108. Otherportions of the system 100 can similarly be implemented using one ormore processing devices of at least one processing platform.

The computer system 101 and the content addressable storage system 105may be implemented on respective distinct processing platforms, althoughnumerous other arrangements are possible. For example, in someembodiments at least portions of the computer system 101 and the contentaddressable storage system 105 are implemented on the same processingplatform. The content addressable storage system 105 can therefore beimplemented at least in part within at least one processing platformthat implements at least a subset of the compute nodes 102.

The term “processing platform” as used herein is intended to be broadlyconstrued so as to encompass, by way of illustration and withoutlimitation, multiple sets of processing devices and associated storagesystems that are configured to communicate over one or more networks.For example, distributed implementations of the system 100 are possible,in which certain components of the system reside in one data center in afirst geographic location while other components of the cluster residein one or more other data centers in one or more other geographiclocations that are potentially remote from the first geographiclocation. Thus, it is possible in some implementations of the system 100for different ones of the compute nodes 102 to reside in different datacenters than the content addressable storage system 105. Numerous otherdistributed implementations of one or both of the computer system 101and the content addressable storage system 105 are possible.Accordingly, the content addressable storage system 105 can also beimplemented in a distributed manner across multiple data centers.

It is to be appreciated that these and other features of illustrativeembodiments are presented by way of example only, and should not beconstrued as limiting in any way.

Accordingly, different numbers, types and arrangements of systemcomponents such as computer system 101, compute nodes 102, network 104,content addressable storage system 105, storage devices 106, storagecontroller 108 and storage nodes 115 and 120 can be used in otherembodiments.

It should be understood that the particular sets of modules and othercomponents implemented in the system 100 as illustrated in FIG. 1 arepresented by way of example only. In other embodiments, only subsets ofthese components, or additional or alternative sets of components, maybe used, and such components may exhibit alternative functionality andconfigurations. For example, as indicated previously, in someillustrative embodiments a given content addressable storage system orother type of storage system with functionality for consistenttermination of data replication across multiple processing modules canbe offered to cloud infrastructure customers or other users as a PaaSoffering.

Additional details of illustrative embodiments will now be describedwith reference to FIGS. 5, 6 and 7. FIGS. 5 and 6 illustrate examples ofinformation processing systems that each include a first contentaddressable storage system such as content addressable storage system105 of the FIG. 1 embodiment that is configured to participate in areplication process with another storage system over at least onenetwork.

In the context of the FIG. 5 embodiment, the storage systemsparticipating in the replication process are assumed to be associatedwith respective source and target sites of the replication process. Forexample, the source site may comprise a production site data center andthe target site may comprise a disaster recovery site data center. TheFIG. 6 embodiment more generally refers to the storage systemsparticipating in the replication process as respective first and secondstorage systems. The first and second storage systems illustrativelycomprise respective content addressable storage systems havingrespective sets of non-volatile memory storage devices, although othertypes of storage systems can be used.

As mentioned previously, FIG. 5 more particularly comprises two separatefigures denoted FIG. 5A and FIG. 5B, each showing different views ofrespective portions of an information processing system.

Referring now to FIG. 5A, an information processing system 500 in anillustrative embodiment comprises a source site data center 502 coupledto at least one network 504. The source site data center 502 comprises astorage system 505 having storage devices 506 and an associated storagecontroller 508. The storage controller 508 comprises replication controllogic 512, snapshot generator 514 and signature generator 516. Thesource site data center 502 further comprises a set of productionservers 519 coupled to the storage system 505.

As indicated above, the storage system 505 in the present embodiment isassumed to comprise a content addressable storage system, although othertypes of storage systems can be used in other embodiments.

The source site data center 502 is coupled via one or more communicationchannels 520 of the network 504 to a target site data center 522 of thesystem 500. The target site data center 522 comprises a storage system525. The storage system 525 comprises storage devices 526 and anassociated storage controller 528. The storage controller 528 comprisesreplication control logic 532, snapshot generator 534 and signaturegenerator 536.

The target site data center 522 further comprises a set of recoveryservers 539 coupled to the storage system 525. The storage system 525,like the storage system 505, is assumed to comprise a contentaddressable storage system, although again other types of storagesystems can be used in other embodiments.

The source site data center 502 and the target site data center 522 areexamples of what are more generally referred to herein as respectiveones of a “source site” and a “target site” of an information processingsystem. The source site data center 502 and the target site data center522 will therefore also be referred to herein as respective source site502 and target site 522 of the system 500. In some embodiments, thetarget site 522 comprises a disaster recovery site data center and thesource site 502 comprises a production site data center, although otherarrangements are possible.

The source site 502 and target site 522 may be implemented in respectivedistinct local and remote geographic locations, although it is alsopossible for the two sites to be within a common facility or evenimplemented on a common processing platform.

It is assumed that data is replicated in system 500 from the source site502 to the target site 522 using a replication process that begins in anasynchronous replication mode, and subsequently transitions from theasynchronous replication mode to a synchronous replication mode. Forexample, the asynchronous replication mode may be used to replicate thebulk of a given set of data from the first storage system to the secondstorage system. The mirroring functionality of the synchronousreplication mode is then enabled. Other arrangements utilizing differentreplication modes and different transitions between the modes arepossible.

The synchronous replication mode in some embodiments is illustrativelyconfigured to mirror data writes between the first and second storagesystems. For example, when a host device writes data to the firststorage system, the first storage system responds to the host devicewith an acknowledgement of successful storage in the first storagesystem only after the first storage system sends the data to the secondstorage system and receives an acknowledgement of successful storageback from the second storage system.

The asynchronous replication mode in some embodiments implementscycle-based asynchronous replication to periodically transfer data inmultiple cycles from the source site 502 to the target site 522. Thedata replicated from the source site 502 to the target site 522 caninclude all of the data stored in the storage system 505, or onlycertain designated subsets of the data stored in the storage system 505.Different replication processes of different types can be implementedfor different parts of the stored data.

A given “replication process” as that term is broadly used herein mayinclude both asynchronous and synchronous replications modes as well assupport for concurrent operation of such modes and separate operation ofthe individual modes. The term “mode” as used herein in conjunction withasynchronous or synchronous replication may therefore itself comprise acorresponding asynchronous or synchronous replication process.

An exemplary cycle-based asynchronous replication process will now bedescribed in more detail. The production servers 519 at the source site502 illustratively run applications for users of the system 500. Theseservers are configured to store application data in the storage system505. This application data is illustratively part of the data stored instorage system 505 that is replicated from the source site 502 to thetarget site 522. The recovery servers 539 at the target site 522 areconfigured to take up the running of the applications for the users ofthe system 500 in the event of a disaster recovery or other recoverysituation. The applications on the recovery servers 539 of the targetsite 522 are started using the data that has been replicated to thetarget site 522 in the cycle-based asynchronous replication process.

The production servers 519 and recovery servers 539 of the respectivesource site 502 and target site 522 illustratively comprise respectiveprocessing devices of one or more processing platforms of thecorresponding source site 502 or target site 522. For example, theseservers can comprise respective VMs each having a processor and amemory, although numerous other configurations are possible. At leastportions of the source site 502 and target site 522 can be implementedin cloud infrastructure such as an AWS system or another cloud-basedsystem such as GCP or Microsoft Azure.

The storage systems 505 and 525 of the source and target sites 502 and522 are configured in the present embodiment for automatic verificationof asynchronously replicated data over multiple cycles of a cycle-basedasynchronous replication process. This illustratively involvesasynchronously replicating data from the storage devices 506 of thestorage system 505 to the storage devices 526 of the storage system 525and automatically verifying the correctness of portions of thereplicated data over multiple cycles.

As noted above, the storage systems 505 and 525 of the source and targetsites 502 and 522 may comprise respective content addressable storagesystems having respective sets of non-volatile memory storage devices.

Additionally or alternatively, the storage systems 505 and 525 of thesource and target sites 502 and 522 may comprise respective clusteredstorage systems having respective sets of storage nodes each having aplurality of storage devices.

In some embodiments, the storage systems 505 and 525 illustrativelycomprise scale-out all-flash storage arrays such as XtremIO™ storagearrays from Dell EMC of Hopkinton, Mass. Other types of storage arrays,including by way of example Unity™, VNX® and Symmetrix VMAX® storagearrays also from Dell EMC, can be used to implement storage systems inother embodiments. A given such storage array can be configured toprovide storage redundancy using well-known RAID techniques such as RAID5 or RAID 6, although other storage redundancy configurations can beused.

The term “storage system” as used herein is therefore intended to bebroadly construed, and should not be viewed as being limited to contentaddressable storage systems or flash-based storage systems.

The storage devices 506 and 526 of respective storage systems 505 and525 illustratively implement a plurality of LUNs configured to storefiles, blocks, objects or other arrangements of data.

In the present embodiment, the storage system 525 of the target site 522is configured to participate in a cycle-based asynchronous replicationprocess with the storage system 505 of the source site 502. Thiscycle-based asynchronous replication process is illustrativelyimplemented in system 500 by cooperative interaction of the storagesystems 505 and 525 over network 504 using their respective replicationcontrol logic 512 and 532, snapshot generators 514 and 534, andsignature generators 516 and 536. Examples of cycles of an illustrativecycle-based asynchronous replication process of this type will bedescribed in more detail below.

The storage system 525 of the target site 522 is more particularlyconfigured in this embodiment to receive from the storage system 505 ofthe source site 502, in respective ones of a plurality of cycles of thecycle-based asynchronous replication process, corresponding sets ofdifferential data representing respective deltas between pairs of sourcesite snapshots for respective pairs of the cycles. The source sitesnapshots are generated by the snapshot generator 514 of the storagecontroller 508.

The storage system 525 of the target site 522 illustratively utilizesthe sets of differential data received in the respective ones of thecycles to update respective target site snapshots for those cycles. Thetarget site snapshots are generated by the snapshot generator 534 of thestorage controller 528.

Over multiple ones of the cycles, the storage system 525 of the targetsite 522 generates target site signatures for respective differentportions of a designated one of the updated target site snapshots. Thetarget site signatures are generated by the signature generator 536 ofthe storage controller 528. The storage system 525 also receives fromthe storage system 505 of the source site 502 corresponding source sitesignatures for respective different portions of a designated one of thesource site snapshots. The source site signatures are generated by thesignature generator 516 of the storage controller 508. The storagesystem 525 compares the target site and source site signatures over themultiple cycles in order to verify that the designated target site andsource site snapshots are equivalent.

Further details regarding asynchronous replication processes suitablefor use in illustrative embodiments herein can be found in U.S. patentapplication Ser. No. 15/662,809, filed Jul. 28, 2017 and entitled“Automatic Verification of Asynchronously Replicated Data,” which isincorporated by reference herein. Other embodiments need not utilizethese automatic verification techniques, and can be implemented usingalternative verification techniques as well as other types ofreplication processes. Accordingly, illustrative embodiments herein arenot limited to use with cycle-based asynchronous replication, but aremore generally applicable to other types of data replication.

The particular exemplary cycle-based asynchronous replication processesdescribed above can be varied in other embodiments. Alternativesynchronous replication processes may also be used. As mentionedpreviously, such processes are performed in respective asynchronous andsynchronous replication modes of a replication process that incorporatesboth asynchronous and synchronous replication.

Each of the source site 502 and target site 522 in the FIG. 5Aembodiment is assumed to be implemented using at least one processingplatform each comprising one or more processing devices each having aprocessor coupled to a memory. Such processing devices canillustratively include particular arrangements of compute, storage andnetwork resources. For example, processing devices in some embodimentsare implemented at least in part utilizing virtual resources such as VMsor LXCs, or combinations of both as in an arrangement in which Dockercontainers or other types of LXCs are configured to run on VMs.

As a more particular example, the storage controllers 508 and 528 orvarious components thereof can each be implemented in the form of one ormore LXCs running on one or more VMs. Other arrangements of one or moreprocessing devices of a processing platform can be used to implement thestorage controllers 508 and 528 and/or their respective components.Other portions of the system 500 can similarly be implemented using oneor more processing devices of at least one processing platform.

The source site 502 and target site 522 are illustratively implementedon respective distinct processing platforms, although numerous otherarrangements are possible. For example, in some embodiments at leastportions of the source site 502 and the target site 522 may beimplemented on the same processing platform. The term “processingplatform” as used herein is intended to be broadly construed so as toencompass, by way of illustration and without limitation, multiple setsof processing devices and associated storage systems that are configuredto communicate over one or more networks.

Referring now to FIG. 5B, a more detailed view of a portion of theinformation processing system 500 is shown, including processing modulesof distributed storage controllers of the source site storage system 505and the target site storage system 525.

As illustrated, a portion of a distributed storage controller of thesource site storage system 505 comprises a plurality of control modules508C-1 through 508C-x and a plurality of routing modules 508R-1 through508R-x. The distributed storage controller of the storage system 505 isassumed to further comprise a plurality of data modules and at least onemanagement module, although these additional processing modules are notshown in the figure for clarity and simplicity of illustration.

Similarly, a portion of a distributed storage controller of the targetsite storage system 525 comprises a plurality of control modules 528C-1through 528C-x and a plurality of routing modules 528R-1 through 528R-x.The distributed storage controller of the storage system 525 is alsoassumed to further comprise a plurality of data modules and at least onemanagement module, although these additional processing modules are notshown in the figure.

Also illustrated in FIG. 5B is a portion of a messaging flow associatedwith a particular host write that is to be replicated from the sourcesite storage system 505 (“source”) to the target site storage system 525(“target”) as part of a synchronous replication process or synchronousreplication mode of the system 500.

The synchronous replication process flow for the given host write inthis embodiment illustratively comprises the following steps:

1. Host write

2. Extent lock at source

3. Write at source

4. Transmit to target

5. Receive in target

6. Extent lock at target

7. Write at target

8. Release extent lock at target

9. Return status to source

10. Update A2H locally at source

11. Release extent lock at source

12. Return status to host

In the figure, steps 1, 2 and 4-6 are illustrated by arrows. The extentlock refers to locking of a particular address range in conjunction withthe host write. The A2H updated in step 10 is an address-to-hash (“A2H”)table that provides a mapping between logical addresses andcorresponding content-based signatures of respective data pages. As thehost write illustratively changes content of one or more such datapages, the content-based signatures and associated A2H table are updatedin conjunction with the host write.

An example of a replication failure condition in this embodiment is afailure of the transmitting control module 508C-1 to receive an expectedresponse from the target site storage system 525 as part of the statusreport in step 9 above indicating that the given host write has beensuccessfully mirrored to the target site storage system 525. Upondetection of such a replication failure condition, the control module508C-1 provides a notification to a management module of the source sitestorage system 505.

The management module then controls the consistent termination of thesynchronous replication process in the manner previously described,through communication with all of the control modules 508C. This causesthe control module 508C-1 to suspend generation of replicationacknowledgements back to the host device that generated the host write.Accordingly, the control module 508C-1 will not return status to thehost in step 12 of the above synchronous replication messaging flow,although it may subsequently acknowledge the host write back to the hostdevice after or otherwise in conjunction with the termination of thesynchronous replication process.

One or more messages associated with returning status back to the hostin step 12 of the synchronous replication messaging flow may thereforebe viewed as an example of what is more generally referred to herein asa “replication acknowledgement.”

The other control modules 508C will operate in a similar manner to thatdescribed above for control module 508C-1, as instructed by themanagement module.

Again, it is to be appreciated that these and other features ofillustrative embodiments are presented by way of example only, andshould not be construed as limiting in any way.

Accordingly, different numbers, types and arrangements of systemcomponents such as source and target sites 502 and 522 and theirrespective storage systems 505 and 525 and storage controllers 508 and528 can be used in other embodiments. In these other embodiments, onlysubsets of these components, or additional or alternative sets ofcomponents, may be used, and such components may exhibit alternativefunctionality and configurations.

The replication process carried out between the source site storagesystem 505 and the target site storage system 525 in the FIG. 5embodiment utilizes consistent termination techniques of the typepreviously described in conjunction with the content addressable storagesystem 105 of FIG. 1. Examples of such consistent terminationarrangements will now be described in further detail with reference toFIGS. 6 and 7.

Turning now to FIG. 6, an information processing system 600 comprises afirst storage system 605 comprising storage devices 606 and adistributed storage controller 608. The distributed storage controller608 comprises a plurality of data modules 608D and a replication engine611 having control logic 612. The data modules 608D implementcompression algorithms 615 for compressing data in conjunction withstorage of the data in the storage devices 606. The replication engine611 and its associated control logic 612 may be implemented at least inpart in one or more control modules and/or management modules of thedistributed storage controller 608, although such modules are notexplicitly shown in the figure. The distributed storage controller 608may be viewed as corresponding to an instance of storage controller 108of FIG. 1 or storage controller 508 or 528 of FIG. 5.

The information processing system 600 further comprises a second storagesystem 625 comprising storage devices 626 and a distributed storagecontroller 628. The distributed storage controller 628 comprises aplurality of data modules 628D and a replication engine 631 havingcontrol logic 632. The data modules 628D implement compressionalgorithms 635 for compressing data in conjunction with storage of thedata in the storage devices 626. The replication engine 631 and itsassociated control logic 632 may be implemented at least in part in oneor more control modules and/or management modules of the distributedstorage controller 628, although such modules are not explicitly shownin the figure. The distributed storage controller 628 may be viewed ascorresponding to an instance of storage controller 108 of FIG. 1 orstorage controller 508 or 528 of FIG. 5.

The compression algorithms 615 and 635 can include any of a number ofwell-known algorithms utilized to compress data in storage systems. Suchalgorithms are therefore not described in detail herein.

In the FIG. 6 embodiment, the first storage system 605 is configured toparticipate in a replication process with the second storage system 625.The replication process is carried out at least in part by thereplication engines 611 and 631 of the respective storage systems 605and 625 as directed by control logic 612 and 632. Such control logic isan example of what is more generally referred to herein as “replicationcontrol logic,” although the latter term is intended to be broadlyconstrued and accordingly in some implementations can encompass anentire replication engine such as replication engine 611 or 631.Replication control logic as disclosed herein can be implemented atleast in the part in the form of software, possibly in combination withassociated hardware and/or firmware.

The data modules 608D of the first storage system 605 are assumed to beconfigured to implement one or more RAID algorithms that involvecompressing data pages in conjunction with storage of the data pages inthe storage devices 606 of the first storage system 605. At least asubset of the data modules 608D are each further assumed to comprise oneor more caches in which data pages are stored in uncompressed form priorto being compressed for storage in the storage devices 606. The datamodules 628D of the second storage system 625 are configured in asimilar manner.

As part of the replication process, the replication engine 611 utilizescontrol logic 612 to request from a given one of the data modules 608Dat least one data page to be replicated to the second storage system625.

For example, replication engine 611 sends a request for one or more datapages, or other type or arrangement of data to be replicated, to theappropriate one of the data modules 608D. The data modules 608D and 628Dare referred to as “backend” data modules in this embodiment relative to“frontend” components such as replication engines 611 and 631 thatcontrol the replication process.

The operation of the information processing system 600 will now bedescribed in further detail with reference to the flow diagram of theillustrative embodiment of FIG. 7. The process as shown includes steps700 through 710, and is suitable for use in the system 600 but is moregenerally applicable to other types of information processing systems,including systems 100 and 500 of respective FIGS. 1 and 5, in whichmultiple storage systems are configured to participate in a replicationprocess. The steps are illustratively performed by cooperativeinteraction of replication engines or other arrangements of replicationcontrol logic of respective storage controllers in respective sourcesite and target site storage systems, also referred to as respectivefirst and second storage systems. A given such storage controller in asource site or target site storage system can comprise a distributedstorage controller implemented in the manner illustrated in FIG. 1, 5 or6.

In step 700, a replication process is initiated between the first andsecond storage systems. The replication process is assumed to comprise asynchronous replication process in which write requests directed by oneor more host devices to the first storage system are mirrored to thesecond storage system. The synchronous replication process may beinitiated responsive to a transition from an asynchronous replicationprocess. For example, the first storage system may initially operate inan asynchronous replication mode and subsequently transition to asynchronous replication mode.

In step 702, in conjunction with the replication process, a controlmodule of the first storage system detects a replication failurecondition for a given write request received from a host device, andprovides a notification of the detected replication failure condition toa system-wide management module of the first storage system. Thereplication failure condition for the given write request illustrativelycomprises a failure to receive in the control module an expectedresponse from the second storage system indicating that the given writerequest has been successfully mirrored to the second storage system.

In step 704, responsive to receipt of the notification of the detectedreplication failure condition, the system-wide management moduleinstructs all control modules to suspend generation of replicationacknowledgments for write requests received from the host device. Inthis embodiment, the term “all control modules” is assumed to includethe particular control module that detected the replication failurecondition, but other embodiments are possible. For example, thesystem-wide management module could alternatively instruct all controlmodules other than the particular control module that detected thereplication failure condition, with that particular control module beingconfigured to perform certain subsequent operations such as suspendinggeneration of replication acknowledgements automatically rather than byexplicit instruction from the system-wide management module.

The control modules in the present embodiment may each be configured toset a replication barrier responsive to receipt of the instruction fromthe system-wide management module to suspend generation of replicationacknowledgments for write requests received from the host device. Othertechniques for suspending generation of replication acknowledgements maybe used in other embodiments.

In step 706, a determination is made as to whether or not confirmationof suspended generation of replication acknowledgments has been receivedfrom all control modules.

In step 708, responsive to receipt of confirmation from all controlmodules, the system-wide management module instructs all control modulesto terminate the replication process. Again, in other embodiments, thesystem-wide management module could alternatively instruct all controlmodules other than the particular control module that detected thereplication failure condition, with that particular control module beingconfigured to perform certain subsequent operations such as replicationtermination automatically rather than by explicit instruction from thesystem-wide management module.

In step 710, the synchronous replication process is terminated. At thispoint, all of the individual control modules have terminated synchronousreplication, and so the overall synchronous replication process is fullyterminated. The first and second storage systems can then transitionback to utilization of an asynchronous replication process. For example,a cycle-based asynchronous replication process may be used aftertermination of the synchronous replication process in step 710.

Additional synchronous replication processes can then be initiated asneeded, through iterated performance of respective instances of the FIG.7 process. The first and second storage systems may therefore beconfigured to transition between asynchronous and synchronousreplication, and vice-versa. During at least a portion of such atransition, the first and second storage systems may concurrentlyoperate in both asynchronous and synchronous replication modes, possiblyusing controlled transition functionality as disclosed in U.S. patentapplication Ser. No. 15/819,666, filed Nov. 21, 2017 and entitled“Storage System Configured for Controlled Transition BetweenAsynchronous and Synchronous Replication Modes,” which is incorporatedby reference herein.

It is also to be appreciated that the FIG. 7 process and other featuresand functionality for consistent termination of data replication acrossmultiple distributed processing modules as described above can beadapted for use with other types of information systems, including byway of example an information processing system in which source site andtarget site storage systems are both implemented on the same processingplatform.

The particular processing operations and other system functionalitydescribed in conjunction with the flow diagram of FIG. 7 are presentedby way of illustrative example only, and should not be construed aslimiting the scope of the disclosure in any way. Alternative embodimentscan use other types of processing operations for implementing consistenttermination of data replication across multiple distributed processingmodules. For example, the ordering of the process steps may be varied inother embodiments, or certain steps may be performed at least in partconcurrently with one another rather than serially. Also, one or more ofthe process steps may be repeated periodically, or multiple instances ofthe process can be performed in parallel with one another in order toimplement a plurality of different consistent termination processes forrespective different sets of replicated data or for different storagesystems or portions thereof within a given information processingsystem.

Functionality such as that described in conjunction with the flowdiagram of FIG. 7 can be implemented at least in part in the form of oneor more software programs stored in memory and executed by a processorof a processing device such as a computer or server. As will bedescribed below, a memory or other storage device having executableprogram code of one or more software programs embodied therein is anexample of what is more generally referred to herein as a“processor-readable storage medium.”

For example, a storage controller such as storage controller 108, 508,528, 608 or 628 that is configured to control performance of one or moresteps of the FIG. 7 process can be implemented as part of what is moregenerally referred to herein as a processing platform comprising one ormore processing devices each comprising a processor coupled to a memory.A given such processing device may correspond to one or more virtualmachines or other types of virtualization infrastructure such as Dockercontainers or other types of LXCs. The storage controller 108, 508, 528,608 or 628, as well as other system components, may be implemented atleast in part using processing devices of such processing platforms. Forexample, in a distributed implementation of the storage controller 108,508, 528, 608 or 628, respective distributed modules of such a storagecontroller can be implemented in respective LXCs running on respectiveones of the processing devices of a processing platform.

In some embodiments, the first and second storage systems compriserespective XtremIO™ storage arrays suitably modified to incorporateconsistent termination techniques as disclosed herein. As describedpreviously, in the context of an XtremIO™ storage array, the controlmodules 108C, data modules 108D, routing modules 108R and managementmodule(s) 108M of the distributed storage controller 108 in system 100illustratively comprise C-modules, D-modules, R-modules and SYMmodule(s), respectively. These exemplary processing modules of thedistributed storage controller 108 can be configured to implementconsistent termination functionality using the FIG. 7 process.

The consistent termination techniques implemented in the embodimentsdescribed above can be varied in other embodiments. For example,different types of process operations can be used in other embodiments.Furthermore, although described in some embodiments in the context ofdata replication from a source to a target, the consistent terminationtechniques in other embodiments can be implemented in the context ofother types of data transfer within a given storage system or from onestorage system to another storage system.

In addition, the above-described functionality associated with C-module,D-module, R-module and SYM module components of an XtremIO™ storagearray can be incorporated into other processing modules or components ofa centralized or distributed storage controller in other types ofstorage systems.

Illustrative embodiments of content addressable storage systems or othertypes of storage systems with functionality for consistent terminationof data replication across multiple distributed processing modules asdisclosed herein can provide a number of significant advantages relativeto conventional arrangements.

For example, some embodiments can advantageously provide highlyefficient termination of a synchronous replication process in thepresence of one or more replication failure conditions in a manner thatautomatically maintains target replica consistency in the presence ofpotentially dependent mirrored host writes. Moreover, such advantagesare provided without adversely impacting system performance.

These and other embodiments include clustered storage systems comprisingstorage controllers that are distributed over multiple storage nodes.Similar advantages can be provided in other types of storage systems.

It is to be appreciated that the particular advantages described aboveand elsewhere herein are associated with particular illustrativeembodiments and need not be present in other embodiments. Also, theparticular types of information processing system features andfunctionality as illustrated in the drawings and described above areexemplary only, and numerous other arrangements may be used in otherembodiments.

As mentioned previously, at least portions of the information processingsystems 100, 500 and 600 may be implemented using one or more processingplatforms. A given such processing platform comprises at least oneprocessing device comprising a processor coupled to a memory. Theprocessor and memory in some embodiments comprise respective processorand memory elements of a virtual machine or container provided using oneor more underlying physical machines. The term “processing device” asused herein is intended to be broadly construed so as to encompass awide variety of different arrangements of physical processors, memoriesand other device components as well as virtual instances of suchcomponents. For example, a “processing device” in some embodiments cancomprise or be executed across one or more virtual processors.Processing devices can therefore be physical or virtual and can beexecuted across one or more physical or virtual processors. It shouldalso be noted that a given virtual device can be mapped to a portion ofa physical one.

Some illustrative embodiments of a processing platform that may be usedto implement at least a portion of an information processing systemcomprise cloud infrastructure including virtual machines implementedusing a hypervisor that runs on physical infrastructure. The cloudinfrastructure further comprises sets of applications running onrespective ones of the virtual machines under the control of thehypervisor. It is also possible to use multiple hypervisors eachproviding a set of virtual machines using at least one underlyingphysical machine. Different sets of virtual machines provided by one ormore hypervisors may be utilized in configuring multiple instances ofvarious components of the system.

These and other types of cloud infrastructure can be used to providewhat is also referred to herein as a multi-tenant environment. One ormore system components such as storage systems 105, 505, 525, 605 and625, or portions thereof, are illustratively implemented for use bytenants of such a multi-tenant environment.

As mentioned previously, cloud infrastructure as disclosed herein caninclude cloud-based systems such as AWS, GCP and Microsoft Azure.Virtual machines provided in such systems can be used to implement atleast portions of one or more of a computer system and a contentaddressable storage system in illustrative embodiments. These and othercloud-based systems in illustrative embodiments can include objectstores such as Amazon S3, GCP Cloud Storage, and Microsoft Azure BlobStorage.

In some embodiments, the cloud infrastructure additionally oralternatively comprises a plurality of containers implemented usingcontainer host devices. For example, a given container of cloudinfrastructure illustratively comprises a Docker container or other typeof LXC. The containers may run on virtual machines in a multi-tenantenvironment, although other arrangements are possible. The containersmay be utilized to implement a variety of different types offunctionality within the system 100, 500 or 600. For example, containerscan be used to implement respective processing devices providing computeand/or storage services of a cloud-based system. Again, containers maybe used in combination with other virtualization infrastructure such asvirtual machines implemented using a hypervisor.

Illustrative embodiments of processing platforms will now be describedin greater detail with reference to FIGS. 8 and 9. Although described inthe context of system 100, these platforms may also be used to implementat least portions of other information processing systems in otherembodiments, such as systems 500 and 600.

FIG. 8 shows an example processing platform comprising cloudinfrastructure 800. The cloud infrastructure 800 comprises a combinationof physical and virtual processing resources that may be utilized toimplement at least a portion of the information processing system 100.The cloud infrastructure 800 comprises virtual machines (VMs) 802-1,802-2, . . . 802-L implemented using a hypervisor 804. The hypervisor804 runs on physical infrastructure 805. The cloud infrastructure 800further comprises sets of applications 810-1, 810-2, . . . 810-L runningon respective ones of the virtual machines 802-1, 802-2, . . . 802-Lunder the control of the hypervisor 804.

Although only a single hypervisor 804 is shown in the embodiment of FIG.8, the system 100 may of course include multiple hypervisors eachproviding a set of virtual machines using at least one underlyingphysical machine. Different sets of virtual machines provided by one ormore hypervisors may be utilized in configuring multiple instances ofvarious components of the system 100.

An example of a commercially available hypervisor platform that may beused to implement hypervisor 804 and possibly other portions of theinformation processing system 100 in one or more embodiments is theVMware® vSphere® which may have an associated virtual infrastructuremanagement system such as the VMware® vCenter™. The underlying physicalmachines may comprise one or more distributed processing platforms thatinclude one or more storage systems.

As is apparent from the above, one or more of the processing modules orother components of system 100 may each run on a computer, server,storage device or other processing platform element. A given suchelement may be viewed as an example of what is more generally referredto herein as a “processing device.” The cloud infrastructure 800 shownin FIG. 8 may represent at least a portion of one processing platform.Another example of such a processing platform is processing platform 900shown in FIG. 9.

The processing platform 900 in this embodiment comprises a portion ofsystem 100 and includes a plurality of processing devices, denoted902-1, 902-2, 902-3, . . . 902-K, which communicate with one anotherover a network 904.

The network 904 may comprise any type of network, including by way ofexample a global computer network such as the Internet, a WAN, a LAN, asatellite network, a telephone or cable network, a cellular network, awireless network such as a WiFi or WiMAX network, or various portions orcombinations of these and other types of networks.

The processing device 902-1 in the processing platform 900 comprises aprocessor 910 coupled to a memory 912.

The processor 910 may comprise a microprocessor, a microcontroller, anapplication-specific integrated circuit (ASIC), a field-programmablegate array (FPGA) or other type of processing circuitry, as well asportions or combinations of such circuitry elements.

The memory 912 may comprise random access memory (RAM), read-only memory(ROM) or other types of memory, in any combination. The memory 912 andother memories disclosed herein should be viewed as illustrativeexamples of what are more generally referred to as “processor-readablestorage media” storing executable program code of one or more softwareprograms.

Articles of manufacture comprising such processor-readable storage mediaare considered illustrative embodiments. A given such article ofmanufacture may comprise, for example, a storage array, a storage diskor an integrated circuit containing RAM, ROM or other electronic memory,or any of a wide variety of other types of computer program products.The term “article of manufacture” as used herein should be understood toexclude transitory, propagating signals. Numerous other types ofcomputer program products comprising processor-readable storage mediacan be used.

Also included in the processing device 902-1 is network interfacecircuitry 914, which is used to interface the processing device with thenetwork 904 and other system components, and may comprise conventionaltransceivers.

The other processing devices 902 of the processing platform 900 areassumed to be configured in a manner similar to that shown forprocessing device 902-1 in the figure.

Again, the particular processing platform 900 shown in the figure ispresented by way of example only, and system 100 may include additionalor alternative processing platforms, as well as numerous distinctprocessing platforms in any combination, with each such platformcomprising one or more computers, servers, storage devices or otherprocessing devices.

For example, other processing platforms used to implement illustrativeembodiments can comprise different types of virtualizationinfrastructure, in place of or in addition to virtualizationinfrastructure comprising virtual machines. Such virtualizationinfrastructure illustratively includes container-based virtualizationinfrastructure configured to provide Docker containers or other types ofLXCs.

As another example, portions of a given processing platform in someembodiments can comprise converged infrastructure such as VxRail™,VxRack™, VxRack™ FLEX, VxBlock™ or Vblock® converged infrastructure fromVCE, the Virtual Computing Environment Company, now the ConvergedPlatform and Solutions Division of Dell EMC.

It should therefore be understood that in other embodiments differentarrangements of additional or alternative elements may be used. At leasta subset of these elements may be collectively implemented on a commonprocessing platform, or each such element may be implemented on aseparate processing platform.

Also, numerous other arrangements of computers, servers, storage devicesor other components are possible in the information processing system100. Such components can communicate with other elements of theinformation processing system 100 over any type of network or othercommunication media.

As indicated previously, components of an information processing systemas disclosed herein can be implemented at least in part in the form ofone or more software programs stored in memory and executed by aprocessor of a processing device. For example, at least portions of thefunctionality of one or more components of the storage controllers 108,508, 528, 608 and 628 of systems 100, 500 and 600 are illustrativelyimplemented in the form of software running on one or more processingdevices.

It should again be emphasized that the above-described embodiments arepresented for purposes of illustration only. Many variations and otheralternative embodiments may be used. For example, the disclosedtechniques are applicable to a wide variety of other types ofinformation processing systems, source and target sites, storagesystems, storage nodes, storage devices, storage controllers,replication processes, replication engines and associated control logic.Also, the particular configurations of system and device elements andassociated processing operations illustratively shown in the drawingscan be varied in other embodiments. Moreover, the various assumptionsmade above in the course of describing the illustrative embodimentsshould also be viewed as exemplary rather than as requirements orlimitations of the disclosure. Numerous other alternative embodimentswithin the scope of the appended claims will be readily apparent tothose skilled in the art.

What is claimed is:
 1. An apparatus comprising: a first storage systemcomprising a plurality of storage nodes; the first storage system beingconfigured to participate in a replication process with a second storagesystem; each of the storage nodes of the first storage system comprisinga plurality of storage devices; each of the storage nodes of the firststorage system further comprising a set of processing modules configuredto communicate over one or more networks with corresponding sets ofprocessing modules on other ones of the storage nodes; the sets ofprocessing modules of the storage nodes collectively comprising at leasta portion of a distributed storage controller of the first storagesystem; wherein in conjunction with the replication process, a first oneof the processing modules is configured to detect a replication failurecondition for a given write request received from a host device, and toprovide a notification to a second one of the processing modules of thedetected replication failure condition; the second processing modulebeing configured, responsive to receipt of the notification of thedetected replication failure condition, to instruct the first processingmodule and a plurality of additional ones of the processing modules of asame type as the first processing module to suspend generation ofreplication acknowledgments for write requests received from the hostdevice; the second processing module being further configured,responsive to receipt of confirmation from the first and additionalprocessing modules of their suspended generation of replicationacknowledgements, to instruct the first and additional processingmodules to terminate the replication process; wherein each of thestorage nodes is implemented using at least one processing devicecomprising a processor coupled to a memory.
 2. The apparatus of claim 1wherein the first and second storage systems comprise respective contentaddressable storage systems having respective sets of non-volatilememory storage devices.
 3. The apparatus of claim 1 wherein the firstand second storage systems are associated with respective source andtarget sites of the replication process and wherein the source sitecomprises a production site data center and the target site comprises adisaster recovery site data center.
 4. The apparatus of claim 1 whereinthe replication process comprises a synchronous replication process inwhich write requests directed by the host device to the first storagesystem are mirrored to the second storage system.
 5. The apparatus ofclaim 1 wherein the replication failure condition for the given writerequest comprises a failure to receive in the first processing module aresponse from the second storage system indicating that the given writerequest has been successfully mirrored to the second storage system. 6.The apparatus of claim 1 wherein the first and additional processingmodules are each configured to set a replication barrier responsive toreceipt of the instruction from the second processing module to suspendgeneration of replication acknowledgments for write requests receivedfrom the host device.
 7. The apparatus of claim 1 wherein: each of thesets of processing modules comprises one or more control modules; atleast one of the sets of processing modules comprises a managementmodule; the first processing module comprises a given one of the controlmodules; the second processing module comprises the management module;and the additional processing modules of the same type as the firstprocessing module comprise respective additional ones of the controlmodules.
 8. The apparatus of claim 7 wherein the management modulecomprises a system-wide management module of the first storage system.9. The apparatus of claim 7 wherein the management module instructs allof the control modules of the distributed storage controller toterminate the replication process only after confirmation of suspendedgeneration of replication acknowledgements is received from each ofthose control modules.
 10. The apparatus of claim 7 wherein themanagement module instructs all of the control modules of thedistributed storage controller to terminate the replication process byinstructing each of the control modules to stop mirroring write requeststo the second storage system.
 11. The apparatus of claim 10 whereindifferent ones of the control modules stop mirroring write requests tothe second storage system at different times.
 12. The apparatus of claim6 wherein each of the first and additional processing modules releasesits set replication barrier in conjunction with termination of thereplication process.
 13. The apparatus of claim 1 wherein the firstprocessing module acknowledges the given write request to the hostdevice in conjunction with termination of the replication process. 14.The apparatus of claim 1 wherein the notification of the detectedreplication failure condition is one of a plurality of suchnotifications received in the second processing module from respectiveones of the first and additional processing modules.
 15. A methodcomprising: configuring a first storage system to include a plurality ofstorage nodes each having a plurality of storage devices, each of thestorage nodes further comprising a set of processing modules configuredto communicate over one or more networks with corresponding sets ofprocessing modules on other ones of the storage nodes; configuring thefirst storage system to participate in a replication process with asecond storage system; and in conjunction with the replication process,a first one of the processing modules detecting a replication failurecondition for a given write request received from a host device, andproviding a notification to a second one of the processing modules ofthe detected replication failure condition; the second processingmodule, responsive to receipt of the notification of the detectedreplication failure condition, instructing the first processing moduleand a plurality of additional ones of the processing modules of a sametype as the first processing module to suspend generation of replicationacknowledgments for write requests received from the host device; thesecond processing module, responsive to receipt of confirmation from thefirst and additional processing modules of their suspended generation ofreplication acknowledgements, instructing the first and additionalprocessing modules to terminate the replication process; wherein themethod is implemented by at least one processing device comprising aprocessor coupled to a memory.
 16. The method of claim 15 wherein thereplication failure condition for the given write request comprises afailure to receive in the first processing module a response from thesecond storage system indicating that the given write request has beensuccessfully mirrored to the second storage system.
 17. The method ofclaim 15 wherein: each of the sets of processing modules comprises oneor more control modules; at least one of the sets of processing modulescomprises a management module; the first processing module comprises agiven one of the control modules; the second processing module comprisesthe management module; and the additional processing modules of the sametype as the first processing module comprise respective additional onesof the control modules.
 18. A computer program product comprising anon-transitory processor-readable storage medium having stored thereinprogram code of one or more software programs, wherein the program codewhen executed by at least one processing device causes said at least oneprocessing device: to configure a first storage system to include aplurality of storage nodes each having a plurality of storage devices,each of the storage nodes further comprising a set of processing modulesconfigured to communicate over one or more networks with correspondingsets of processing modules on other ones of the storage nodes; toconfigure the first storage system to participate in a replicationprocess with a second storage system; and in conjunction with thereplication process, a first one of the processing modules beingconfigured to detect a replication failure condition for a given writerequest received from a host device, and to provide a notification to asecond one of the processing modules of the detected replication failurecondition; the second processing module being configured, responsive toreceipt of the notification of the detected replication failurecondition, to instruct the first processing module and a plurality ofadditional ones of the processing modules of a same type as the firstprocessing module to suspend generation of replication acknowledgmentsfor write requests received from the host device; the second processingmodule being further configured, responsive to receipt of confirmationfrom the first and additional processing modules of their suspendedgeneration of replication acknowledgements, to instruct the first andadditional processing modules to terminate the replication process. 19.The computer program product of claim 18 wherein the replication failurecondition for the given write request comprises a failure to receive inthe first processing module a response from the second storage systemindicating that the given write request has been successfully mirroredto the second storage system.
 20. The computer program product of claim18 wherein: each of the sets of processing modules comprises one or morecontrol modules; at least one of the sets of processing modulescomprises a management module; the first processing module comprises agiven one of the control modules; the second processing module comprisesthe management module; and the additional processing modules of the sametype as the first processing module comprise respective additional onesof the control modules.